Semiconductor device coding using quantum dot technology

ABSTRACT

Semiconductor device identification using quantum dot technology. A semiconductor nanocrystal based target is fabricated. A guard ring superjacent the fluorescing surface of the nanocrystal surface is provided to ensure repeatability of spectral mapping and analysis data. A transparent cap on the target may enhance performance. A system for coding a semiconductor device is described. A method is described for fabricating quantum dot targets in a methodology compatible with subsequent semiconductor fabrication process steps.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO AN APPENDIX

Not applicable.

FIELD

The technology described herein is generally related to the field ofsemiconductor devices and to the use of quantum dot technology toprovide an integral, unique, reproducible identifier to be associatedwith each device.

BACKGROUND

A variety of technologies are used for semiconductor deviceidentification, recognition, authentication, verification, security, andthe like; for simplicity of explanation of the present invention, suchissues and processes and mechanisms associated with such issues aregenerically referred to generically as “coding.” For example, to resolvesuch issues in semiconductor devices—e.g., integrated circuits (ICs),ink-jet printheads, nanomachines, and the like—they are often codedusing mechanisms such as bar codes, distinct pattern lithographicfeatures, encryption circuitry, tamper detection circuitry, oractivation firmware programming, and the like.

Quantum dots are semiconductor crystals having a size on the order ofjust a few nanometers. Known manner fabrication of quantum dotconstructs is described in the textbook titled “Quantum DotHeterostructures,” by D. Bimberg, M. Grundmann, and N. N. Ledentsov,copyright, John Wiley & Sons, U.K. publishers, 1998, and in U.S. Pat.No. 6,942,731 by Sellin et al. (including Bimberg, D.), titled “MethodFor Improving The Efficiency Of Epitaxially Produced Quantum DotSemiconductor Components,” each incorporated herein by reference.

Quantum dots structures, as in the case of the present invention, may beself-assembling, self-organized constructs that are characterized byelectron, hole, or electron-hole pair confinement which results indiscrete quantized energy levels and distinct spectralphotoluminescence. The larger a quantum dot, the more towards the redend of the spectrum the fluorescence; the smaller the dot, the moretowards the blue end. In other words, a quantum dot construct comprisingmany nanocrystals will exhibit a distinct luminescence spectral emissionwhen reflected light is analyzed by known manner spectroscopytechniques. Because of the heterogenous nature of each quantum dotstructure, each will have a unique spectral pattern, analogous to a“signature” or “fingerprint.” See e.g., “Near-Field Spectroscopy of theQuantum Constituents of a Luminescent System,” H. F. Hess et al.,Science, vol. 264, 1994, pp. 1740.

In U.S. Pat. No. 6,633,370, for “Quantum Dots, SemiconductorNanocrystals and Semiconductor Particles Used As Fluorescent CodingElements,” N. M. Lawandy discusses impediments to using quantum dots forcoding, labeling and authentication applications for integratedcircuits. Lawandy instead proposes using “semiconductor particles havinga radius larger than a quantum dot radius for a corresponding material.”

With semiconductor devices having an ever increasing complexity,continual miniaturization of the discrete components therein, and issuesregarding counterfeit products, there is a need for improvedsemiconductor device coding technologies.

SUMMARY

The present invention generally provides for using quantum dottechnology for semiconductor device coding.

In one aspect, the present invention provides an apparatus for coding asemiconductor device, the apparatus including: formed on said device,targeting mechanisms for emitting quantum dot constructphotoluminescence; and associated with said targeting mechanisms,masking mechanisms for defining an outer periphery of said targetingmechanisms.

In another aspect, the present invention provides a system for coding asemiconductor substrate, the system including: on said substrate, atleast one quantum dot target having a predetermined chemistry andemitting a substantially repeatable photoluminescence; positioned forilluminating each said target, an illumination source havingphotoemissions of a predetermined wavelength associated with saidpredetermined chemistry for instigating said photoluminescence; adetector positioned for receiving said photoluminescence instigated bysaid illumination source; associated with said detector, aphotoluminescence reading and mapping subsystem for generating datarecords from said quantum dot photoluminescence; and a data processorfor storing maps associated with said data records and for comparingsaid maps to subsequently received data records associated withsubsequent photoluminescence emissions instigated by said illuminationsource.

In another aspect, the present invention provides a method for forming aquantum dot coding apparatus on a semiconductor device having a givensubstrate using known manner fabrication processes associated with thechemistry of said given substrate, the method having steps including:forming an etch stop layer across an active element surface of saidsubstrate; forming a semiconductor nanocrystal layer comprising quantumdots superjacent said etch stop layer; forming at least one targetingmask superjacent said semiconductor nanocrystal layer; removing regionsof said nanocrystal layer not subjacent each targeting mask; forming avia in said targeting mask, exposing a surface region of saidnanocrystal layer; and forming a passivation layer superjacent saidtargeting mask and said surface region of said nanocrystal layer,wherein said mask and surface region of said nanocrystal layer form atarget for impinging light for causing photoluminescence from saidsurface region of said nanocrystal layer through said via, and whereinsaid passivation layer protects said target from subsequent process forforming active elements of said semiconductor device in said substrate.

Some objects and advantages of the present invention are:

self-assembled quantum dot constructs have good crystalline quality anda protective interface;

it is relatively simple as compared with other authentication orverification techniques;

it has no electrical nor optical functions related to the device uponwhich it is mounted and therefore does not affect performance of device;

associated identity authentication, or decoding, processes do not dependon detection of a response which is related to functions of the deviceupon which it is attached;

because growth temperature of quantum dot structures is generally higherthan known manner semiconductor process temperature, and because ofthree dimensional confinement (less temperature sensitivity), quantumdot structures are stable and are not expected to change with time;

physical size of quantum dot structures make them invisible to theunaided-eye and difficult to detect even with the aid of systems such asscanning electron microscopes; and

in operation, identity authentication tests do not need any bias norneed to be triggered, no modification to response in detection isneeded, and physical attacks can be sensed and verified.

The foregoing summary is not intended to be inclusive of all aspects,objects, advantages and features of the present invention nor should anylimitation on the scope of the invention be implied therefrom. ThisBrief Summary is provided in accordance with the mandate of 37 C.F.R.1.73 and M.P.E.P. 608.01(d) merely to apprise the public, and moreespecially those interested in the particular art to which the inventionrelates, of the nature of the invention in order to be of assistance inaiding ready understanding of the patent in future searches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 1A in accordance with an exemplary embodiment of the presentinvention is a schematic diagram depicting a starting structure andprocess step.

FIGS. 2 a and 2 b in accordance with the exemplary embodiment of FIGS. 1and 1A depict continuing process steps and resultant structure.

FIGS. 3 a and 3 b in accordance with the exemplary embodiment of FIGS.1, 1A, 2 a and 2 b depict continuing process steps and resultantstructure.

FIG. 4 is a schematic block diagram of another exemplary embodiment ofthe present invention; overhead view.

FIG. 5 is a schematic block diagram of a system implementation exemplaryembodiment in accordance with the present invention.

Like reference designations represent like features throughout thedrawings. The drawings in this specification should be understood as notbeing drawn to scale unless specifically annotated as such.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In general, the present invention provides for quantum dot targetconstructs that may be associated with semiconductor devices for thepurpose of providing each of the devices with a unique, identifiable,code or set of codes. For convenience of explanation, the presentinvention is described in conjunction with the fabrication of an IC chipexemplary embodiment. However, it will be recognized by those skilled inthe art that the invention may be practiced in conjunction with thefabrication of any semiconductor device in which the processes arecompatible with the formation of quantum dot constructs. No limitationon the scope of the invention is intended by exemplary embodimentdescriptions nor should any be implied therefrom. Standard periodictable symbols and integrated circuit symbols commonly understood bythose skilled in the art are used throughout in the description.

FIG. 1 depicts a first stage of fabrication of a construct in accordancewith the present invention in which an exemplary semiconductor substrate101 such as would be used to form heterojunction bipolar transistors hasbeen formed via known manner molecular beam epitaxy. Process andstructures of this type are described in a text titled “InP HBTs:Growth, Processing, and Applications,” by Artech House, Norwood MA,copyright 1995, Jalali, Peaston. ed., incorporated herein by reference.For this exemplary embodiment, a (100) InP substrate 101—such as a shownexemplary layer stack of a heterojunction bipolar transistor device—isemployed. An etch stop layer 103—e.g., InP—is formed in a known manneron surface 102 of the substrate 101. Superjacent the etch stop layer103, a quantum dot layer 105 is formed (see Background; re Bimberg andSellin et al.).

One quantum dot layer 105 is shown—depicted schematically in FIG. 1 asan idealized monolayer having a height of approximately one nanocrystal107, 109, 111, 113. It should be recognized that providing a pluralityof layers may be more practical, depending in part upon thesophistication of the methods and apparatus employed in forming thequantum dot construct and in mapping and analyzing a quantum dot regionsphotoluminescence spectrum code. FIG. 1A schematically depicts a quantumdot layered structure using a barrier. In an experimental exemplaryembodiment, InAs quantum dot layers 105 were formed as a stack of a fewmonolayers 105 of InAs, embedded in InAlGaAs or GaInAsP barrier layers104 a, 104 b, preferably with cap layers (not shown in these FIGs; see,Bimberg and Sellin et al., supra, regarding known manner techniqueswhich may be employed in conjunction with the present invention). Ifsuch stacked monolayers are employed, each barrier layer 104 a, 104 bcan be equal to or greater than 250 Angstroms. In general, otherspecific implementations of self-assembled, self-organized, quantum dotlayer(s) for coding a device based on a distinct photoluminescencespectra characteristics of quantum dots and compatible withsemiconductor processes for forming the underlying device may beselected. Moreover, specially designed vertically aligned or stackedhigh density quantum dot structures that are consistent with a specificsemiconductor device technology can be associated in a variety ofimplementations.

FIGS. 2 a and 2 b depict a next stage of fabrication and structure inwhich a targeting mask 201 is formed adjacent to the quantum dot layer105. In this embodiment, the targeting mask 201 is exemplified as ametal layer ring formed in a known manner over at least one surfaceregion of the quantum dot layer 105. An aperture 207 is formed in thetargeting mask 201. The outer diameter 203 and inner diameter 205 of thering can be determined in accordance with current photolithography orother known manner semiconductor device fabrication techniques. Theaperture 207 is formed in this exemplary embodiment with an innerdiameter 205 of an exemplary circular mask 201. The shapes (open orclosed) and dimensions of the aperture 207 may be selected for aspecific implementation such that the exposed quantum dot (“QDs”) layer105 region 107 subjacent the aperture supports the need of thephotoluminescent spectrum to be mapped and analyzed for ID purposes (seeFIG. 5 and related description below). This will be related to the sizeand density of the quantum dots which is determined by the chemistry ofthe materials employed in forming the quantum dot layer 105. It alsoshould be recognized that while in this exemplary embodiment thetargeting mask 201 was formed superjacent to the quantum dot layer 105,in other implementations it may be convenient to embed such a mask,forming a target where the surface of the quantum dot region and thesurface of the mask are semi-planar or substantially coplanar.

Furthermore, a plurality of targeting masks 201 may be formedsimultaneously, for example with respect to more than one of the quantumdot layer crystal regions 107 109, 111, and 113 as depicted in FIG. 1.It may be advantageous in some implementations to provide a set oftargets—namely, a set of identity signature quantum dot targetconstructs—for each device to be scribed from a semiconductor wafer (notshown). In other words, each device would have a set of unique codes,analogous to a set of fingerprints in which one may be sufficient toconfirm true identity. In this manner, in the event that one or more ofthe target constructs becomes defective or disabled, another of theconstructs may still be mapped and analyzed with respect to its uniquephotoluminescent spectrum.

In general it is believed that the smaller the aperture 207, the sharperthe photoluminescent spectrum lines will be. In this exemplaryembodiment and in accordance with current state-of-the-art fabricationtechniques, the inner diameter 205 of the ring is submicron. A purposeof the targeting mask 201 is to ensure repeatability in performance ofthe present invention. It is known that quantum dots have threedimensional confinements within dimensions smaller than their deBrogliewavelength. Narrow and specific photoluminescence emission peaks haveintensities depending in part upon excitation wavelength. The targetingmask 201 ensures a more consistent illumination factor and fluorescencein that generally the same specific quantum dots will fluoresce.

FIGS. 3 a and 3 b depict the formation of discrete quantum dot targets.In FIG. 3 a, the unused regions of the quantum dot layer 105 have beenremoved, namely all volumes not subjacent targeting mask 201. Forexample, an inductively-coupled plasma etch (ICP: may have an end pointsystem with optical emission intensities) using a Cl-based ormethane-hydrogen based chemistry, may be employed; wet chemistry methodsalso may be employed. This leaves a quantum dot target 301 at one, ormore, known positions on the surface of the etch stop layer 103 of thesubstrate 101. Note that these steps simultaneously exposed theheterojunction bipolar transistor layers (labeled) for known mannerintegrated circuit processing to form discrete elements therefrom.Further description of those steps are not material to an understandingof the present invention.

Turning to FIG. 3 b, a target protection cap 303—such as anincoming-wavelength-transparent silicon nitride in this exemplaryembodiment—is formed on top of the quantum dot target(s) 301, coveringthe mask and the quantum dot fluorescing surface. This protects thequantum dot target(s) 301 during the remainder of the heterojunctionbipolar transistor fabrication steps. In this Figure, an exemplaryemitter metal (EM) contact 311 not shown in previous Figures was formedin a known manner on top of the Emitter cap layer and discrete subjacentN-Emitter mesas were formed by known manner to expose the base layer317. Continuing known manner steps may be used to complete the ICconstructs; again, further explanation is not material to anunderstanding of the present invention. What is material is that thetarget 301 protection cap 303, when formed of silicon nitride (Si3N4),has been found to enhance performance of the present invention.

In an experimental implementation, an Si3N4 cap 303 having a thicknessof approximately 500 Angstroms was grown on an Sb4122 quantum dotcontrol material. Photoluminescence was measured from the control piece,the Si3N4 capped control piece, and a piece of bulk InAs. In comparison,the peak position appeared unchanged in the Si3N4 capped control and theintensity appeared to be increased. It is believed that the Si3N4 capacts as an antireflective coating. Thus, the target protection cap 303(FIG. 3 b) was found by the inventors to enhance performance in thepresent invention. Other transparent materials, e.g., silicon dioxide(Si02), are expected to act similarly; experimentation may yield furtherimprovements.

FIG. 4 is another implementation of an exemplary embodiment of thepresent invention is shown. In this aerial view, an IC chip 401 isschematically depicted, having a plurality of functional components 403(labeled). The IC chip 401 is shown to have a quantum dot target 301.Other quantum dot targets may be fabricated, shown as phantom linecircles 301A, at various locations in conformance with the processesassociated with fabricating each component 403 or simultaneously for theentire chip 401. The exact location(s) of the target(s) 301, 301A withrespect to other components on the chip 401 may be kept secret as asecurity measure. An illumination source and detector (see, FIG. 5 anddetailed description below) having a narrow field-of-view will capturethe fixed photoluminescence signature(s) of each target. However, italso should be recognized that high-resolution systems can be designedwith end-users' input. Therefore, advancements in photoluminescencemapping systems has the potential of meeting requirements for mappinghigh-resolution quantum dot emission spectra.

FIG. 5 depicts a block diagram of a system for mapping and analyzingphotoluminescent data from a semiconductor device 501 having at leastone quantum dot target 301 thereon (three shown). An illumination source503, such as a known manner GaAs diode is used to project light(depicted by wavy lines) narrow beam, scanning or the like, onto thedevice 501, impinging on each target 301, repeatably as substantiallyensured by use of the targeting mask feature. Narrow and specificemission peaks with intensity depending on excitation wavelength is animportant characteristic of a fluorescing quantum dot target 301. Aphotoluminescence mapper 505 in combination with a data processor 507can both record location of the device 501 associated quantum dottargets 301 and map, record each map, and analyze the photoluminescencespectra, as well as intensities, line shapes, line widths, and number ofpeaks of photoluminescence peaks vs. wavelength.

More particularly, there are a variety of factors and characteristicsmaking quantum dot technology highly suited to coding in accordance withthe present invention. Photoluminescence wavelength is sensitive topatterns, size, height and density of quantum dot targets 301. Forexample, quantum dot density may make its spectrum facet dependent; somefacets can be denser than others. This characteristic may allow quantumdot targets 301 with various density formed at various facets (bypatterning, etching and revealing the desired facets at designated area)to be formed on same wafer. The photoluminescence spectra of the quantumdot targets 301 with various densities may be distinct from one locationto another on a wafer. Photoluminescence peak energy and quantum dotcrystal size distribution are found dependent strongly on the growthcondition—e.g., substrate temperature, growth rate, V/III ratio, growthrate, and the like—of the layer used to form the quantum dot layers,including any barrier layers employed. Photoluminescence peak energy candepend on thickness of the dot, as well as strain and shape of the dots.Quantum dot photoluminescence peak width is generally also found relatedto distribution in height of the dots. Photoluminescence peak line widthreduction can be obtained from smaller distribution in dot height.InAs/InP quantum dot formation, height dispersion control was found tobe making photoluminescence line widths narrower. Island size reductionincreases quantum confinement was found from InAs/InP quantum dots ofthe exemplary embodiment, demonstrated through room temperaturephotoluminescence spectrum analysis. Vertical electronic coupling wasfound between InAs/InP quantum dots grown on (113) B InP; and on closelystacked InAs/GaAs quantum dots grown at slow growth rate.Photoluminescence spectrum of ordered arrays of quantum dots isdominated by a single sharp line, while series of sharp lines indicatetransitions in quantum dots of different sizes. However several sizes ofdots can be combined to create an almost infinite variety of emissionspectra. By changing the number of dots, their individualconcentrations, their emission peaks and for their excitationwavelength, a vast code can be designed and inserted into semiconductormaterials. Quite often etch-revealed planes are involved in quantum dotand quantum well structures. Lower index plane is more common. Quantumdots on higher index planes in both material systems were found ofimproved quantum dots size uniformity. In applications for semiconductorIC authentication selectively placed self-assembled quantum dot targets301 on various index facets can potentially provide more intentional,controlled quantum dots density variation and hence intentionalphotoluminescence spectra variations from location to location.Dependence of the photoluminescence spectrum on growth parameters, aswell as detailed layer structures can be found through experiments. Theresults can point out directions for optimizing quantum dots quality andlayer structures of quantum dots part for obtaining desired/preferredphotoluminescence spectra fingerprints for semiconductor device coding.

The recorded “maps” define the unique virtual “signature” or“fingerprint” of each target 301. A commercially available mappingsubsystem may be employed, such as described by RPMSigma in CompoundSemiconductor Magazine, product release announcement September 2005:“Accent: PL Mapping Product” and “Accent: PL Mapping Software Features.”Low temperature (vs. room temperature) photoluminescence mapping systemswith submicron resolution and scan area of 25×25 mm are also known andmay be employed; see e.g., M. De Vittorio et al, Review of ScientificInstruments, vol. 72, no. 6, 2001, pp. 2610. Studies from III/V arsenidequantum dot constructs have demonstrated that the exciton recombinationtimes and exciton dephasing times are in the nanosecond range andhundreds of picoseconds; much longer than needed in takingphotoluminescence spectrum data.

Next, this record of quantum dot target photoluminescence spectra datais stored in a data processing subsystem 507. In other words, the storedrecord has become a unique signature, permanently associated with thedevice 501.

At a later time, the record can be accessed and a currently unidentifiedor suspect device placed in the system 501 can be illuminated and itsphotoluminescence spectra can be mapped and compared to the record todetermine whether there is a match. A match-intended fingerprint will behard to duplicate. Thus, because of its unique photoluminescencespectrum, quantum dots target(s) would enable semiconductorauthentication by allowing each device to have a distinct signaturebased on the attached quantum dots target specific photoluminescencespectrum.

From the foregoing description, it will be apparent that the presentinvention has a number of advantages, some of which have been describedabove, and others of which are inherent in the embodiments of theinvention described above. Also, it will be understood thatmodifications can be made to the invention described without departingfrom the teachings of subject matter described herein. As such, theinvention is not to be limited to the described embodiments except asrequired by the appended claims.

The invention claimed is:
 1. An apparatus for coding a semiconductordevice, the apparatus comprising: targeting means in said semiconductordevice configured for emitting quantum dot construct photoluminescencefor coding the semiconductor device when illuminated; associated withsaid targeting means, masking means formed adjacent to said targetingmeans; wherein the masking means is metal; wherein said masking meansdefines an aperture; and wherein said targeting means is configured sothat when a photoluminescence surface of said targeting means is exposedto illumination through said aperture, said targeting means emitsphotoluminescence through said aperture; and wherein said masking meanscomprises an opaque material having a shape and dimensions predeterminedfor directing light from a source onto said targeting means such thatsaid photoluminescence presents a repeatable spectrum emitted from saidtargeting means.
 2. The apparatus as set forth in claim 1, said maskingmeans comprising: an opaque material defining an aperture wherein afluorescing surface of said targeting means is exposed through saidaperture.
 3. The apparatus as set forth in claim 1, further comprising:capping means for protecting a fluorescing surface of said targetingmeans.
 4. The apparatus as set forth in claim 3 wherein said cappingmeans is formed of a transparent material for enhancing fluorescentemissions from said targeting means.
 5. The apparatus as set forth inclaim 1 wherein said targeting means comprises: a semiconductornanocrystal quantum dot construct compatible with processes used information of active elements of said semiconductor device.
 6. Theapparatus as set forth in claim 1, wherein said targeting meanscomprises a quantum dot.
 7. The apparatus as set forth in claim 1,wherein said targeting means comprises a plurality of quantum dots. 8.The apparatus as set forth in claim 1, wherein said masking means isformed superjacent to said targeting means.
 9. The apparatus as setforth in claim 1, wherein said masking means is formed semi-planar orsubstantially coplanar to said targeting means.
 10. The system of claim1 wherein said masking means is a metal ring.
 11. The system of claim 1wherein said masking means is not in direct contact with the targetingmeans.
 12. A system for quantum dot coding on a semiconductor devicecomprising: a semiconductor nanocrystal layer in said semiconductordevice configured for coding the semiconductor device when illuminatedcomprising at least one quantum dot; at least one targeting masksuperjacent said semiconductor nanocrystal layer, the targeting maskhaving an aperture exposing said quantum dot in said nanocrystal layer;wherein the targeting mask is metal; and wherein the semiconductornanocrystal layer is configured so that impinging light through saidaperture of said targeting mask causes photoluminescence from said atleast one quantum dot to be emitted through said aperture; and whereinsaid targeting mask comprises an opaque material having a shape anddimensions predetermined for directing impinging from a source onto saidat least one quantum dot such that said photoluminescence presents arepeatable spectrum emitted from said at least one quantum dot.
 13. Thesystem of claim 12 further comprising: an etch stop layer across anactive element surface of a substrate of said semiconductor device;wherein said semiconductor nanocrystal layer is superjacent said etchstop layer.
 14. The system of claim 12 further comprising: a passivationlayer superjacent said targeting mask; wherein said passivation layerprotects said targeting mask from processes for forming active elementsof said semiconductor device in said substrate.
 15. The system of claim14, wherein said passivation layer comprises a transparent material. 16.The system of claim 12 wherein said targeting mask is a metal ring. 17.The system of claim 12 wherein said targeting mask is not in directcontact with the quantum dot.